The paper deals with a derivation of simple and efficient analytical solution for the calculation of transient impedance of the horizontal grounding electrode. The Generalized Telegrapher’s Equation for spatial distribution of the scattered voltage in the frequency domain is used to derive the transient voltage at the feed point of the grounding electrode. Solution is obtained by using the Inverse Laplace Transform and Cauchy residue theorem. Transient impedance is determined as a ratio of time domain voltage and (lightning) current at the feeding point. Some illustrative numerical results are given.